Connected: An Internet Encyclopedia
RFC 821 - 4.2.1. REPLY CODES BY FUNCTION GROUPS

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### 4.2.1. REPLY CODES BY FUNCTION GROUPS

             500 Syntax error, command unrecognized
                [This may include errors such as command line too long]
             501 Syntax error in parameters or arguments
             502 Command not implemented
             503 Bad sequence of commands
             504 Command parameter not implemented

             211 System status, or system help reply
             214 Help message
                [Information on how to use the receiver or the meaning of a
                particular non-standard command; this reply is useful only
                to the human user]

             220 <domain> Service ready
             221 <domain> Service closing transmission channel
             421 <domain> Service not available,
                 closing transmission channel
                [This may be a reply to any command if the service knows it
                must shut down]

             250 Requested mail action okay, completed
             251 User not local; will forward to <forward-path>
             450 Requested mail action not taken: mailbox unavailable
                [E.g., mailbox busy]
             550 Requested action not taken: mailbox unavailable
                [E.g., mailbox not found, no access]
             451 Requested action aborted: error in processing
             551 User not local; please try <forward-path>
             452 Requested action not taken: insufficient system storage
             552 Requested mail action aborted: exceeded storage allocation
             553 Requested action not taken: mailbox name not allowed
                [E.g., mailbox syntax incorrect]
             354 Start mail input; end with <CRLF>.<CRLF>
             554 Transaction failed

Next: 4.2.2. NUMERIC ORDER LIST OF REPLY CODES

Connected: An Internet Encyclopedia
RFC 821 - 4.2.1. REPLY CODES BY FUNCTION GROUPS